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Unit 06

COMPOSITION AND STRUCTURE OF THE ATMOSPHERE

Unit Overview

This unit discusses the atmosphere, which is one of our planet's major spheres. The main sections are:

Contents of the atmosphere

The layered structure of the atmosphere

Research Frontiers

The atmosphere consists of the gases (both constant and variable), droplets, and particles surrounding the Earth's surface. Three constant gases - nitrogen, oxygen, and argon - account for 78.1%, 21%, and 0.9%, respectively, of the atmosphere's volume. Although variable gases comprise a much smaller percentage of the atmosphere's volume, they are very important with respect to the "greenhouse effect," precipitation, and the absorption of ultraviolet radiation. Water vapour and carbon dioxide are the two most important "greenhouse" gases; water vapour is also a precursor to clouds and precipitation.

Another "greenhouse" gas - ozone - also protects living organisms from ultraviolet radiation but is a major atmospheric pollutant. Particles in the atmosphere can also modify the planetary energy balance (e.g., generally lead to a cooling of the Earth's surface); they are also important atmospheric pollutants.

The atmosphere can be divided into the following generalized layers: troposphere, stratosphere, mesosphere, and thermosphere. Humans and other organisms live in the troposphere, while the state of the other layers impacts the welfare of humans, and of the planet as a whole.

Unit Objectives

To describe the constituents of the atmosphere and their relative concentrations

To survey the four layers of the atmosphere together with their major properties

To discuss the problem of ozone depletion and its consequences

Glossary of Key Terms

Aerosols

Tiny solid or liquid particles suspended in the atmosphere.

Carbon dioxide cycle

Dominated by exchanges occurring between the air and the sea; carbon
dioxide is directly absorbed by the ocean from the atmosphere and is released
during the photosynthesis of billions of small organisms known as plankton.

Chlorofluorocarbons (CFCs)

Ozone-depleting chemicals widely used as coolants, propellents,
cleaning solvents, and as components in plastic foam prior to 1990.

Climate

The long- term conditions (over at least 30 years) of aggregate weather over a region, summarized by averages and measures of variability; a synthesis of the succession of weather events we have learned to expect at any given location.

Constant gases

Atmospheric gases always found in the same proportions; two of them constitute over 99 percent of the air, nitrogen (78 percent) and oxygen (21 percent).

Hydrologic cycle

The complex system of exchange involving water in its various forms as it continually circulates among the atmosphere, lithosphere, hydrosphere, biosphere, and cryosphere.

Impurities

Solid particles floating in the atmosphere whose quantities vary in time and
space; among other things, they play an active role in the formation of raindrops.

Lapse rate

The rate of decline in temperature as altitude increases; the average lapse rate of temperature with height in the troposphere is 0.65°C/100 m (3.5°F/1000 ft).

Mesopause

The upper boundary of the mesosphere, lying approximately 80 km (50 mi) above the surface.

Mesosphere

The third layer of the atmosphere, lying above the troposphere and stratosphere; here temperatures again decline with increasing elevation as they do in the troposphere.

Nitrogen cycle

Cycle in which nitrogen-fixing bacteria convert atmospheric nitrogen into the organic compounds in plants. This organic material is consumed by animals (including humans) when the plants are eaten. When animals die, organic proteins are transformed by other bacteria and microorganisms to ammonia, urea, nitrates, and finally back into gaseous nitrogen.

Oxygen cycle

Oxygen is put back onto the atmosphere as a by-product of photosynthesis, and is lost when it is inhaled by animals or chemically combined with other materials during oxidation.

Ozone layer

Also known as the ozonosphere, the ozone-rich layer of the atmosphere that extends between 15 and 50 km (9 and 31 mi) above the surface; the highest concentrations of ozone are usually found at the level between 20 and 25 km (12 and 15 mi).

Stratopause

The upper boundary of the stratosphere, lying approximately 52 Ian (32mi) above the surface.

Stratosphere

The atmospheric layer lying above the troposphere; here temperatures are
either constant or start increasing with altitude.

Temperature inversion

Condition in which temperature increases with altitude rather than
decreases - a positive lapse rate; it inverts what we, on the surface, believe to be the "normal" behavior of temperature change with increasing height.

Thermosphere

The fourth layer of the atmosphere, lying respectively above the troposphere, stratosphere, and mesosphere; in this layer, temperatures increase as altitude increases.

Tropopause

The upper boundary of the troposphere along which temperatures stop decreasing with height.

Troposphere

The bottom layer of the atmosphere in which temperature usually decreases with altitude.

Variable gases

Atmospheric gases present in differing quantities at different times and places; three are essential to human well-being: carbon dioxide, water vapour, and ozone.

Water vapour

The invisible gaseous form of water; the most widely distributed variable
gas of the atmosphere.

Weather

The immediate and short-term conditions of the atmosphere that impinge on
daily human activities.